1. Field of the Invention
This invention relates generally to the field of acoustic signal generators of the type wherein an impact mass is propelled forceably downward against a rigid target in contact with the earth. More particularly, the invention is concerned with the construction of such targets.
2. Description of the Prior Art
Prior art seismic impulse generators of the character referenced above are typically provided with means for propelling an impact mass into contact with a rigid earth-contacting target in axial alignment with the moving mass. The attained kinetic energy of the impact mass is transferred instantaneously into the target. Generally speaking, the target is given a relatively wide bottom area in contact with the earth so as to distribute the force of the impact over a fairly wide area. This helps to insure that the underlying earth is subjected to an elastic rather than a plastic deformation so that the available energy is utilized with maximum efficiency in creating an acoustic pulse to be injected into the earth. For rapid acceleration, the target element is preferably light in weight and, of course, is sufficiently rigid to avoid bending or bowing in the middle. Generally speaking, the target has has a flat bottom plate and is provided with an upstanding pillar or hub portion adapted to receive a blow from the impact mass. The target may either be a separate element or may be interfitted within the structure which guides the path of the impact mass. The latter arrangement is useful, not only for purposes of alignment but also makes it relatively easy to pick up the target in moving from one site to another.
In devices of this character, a high speed impact mass accelerates the target downwardly at a corresponding high rate. This rate determines the frequency spectrum of the resultant acoustic signal injected into the earth. If one wishes to change the character of the injected signal such as by eliminating the highest frequency components, the prior art teaches that this is accomplished by reducing in some manner the velocity of the impact mass or increasing the weight of the target.
It is, therefore, a general object of this invention to provide a target for an impulsive-type acoustic signal generator with improved means for controlling the frequency characteristics of the injected acoustic signal without sacrificing energy efficiency.
A further disadvantage which has been noted in the operation of an impulsive-type acoustic signal generator employing such a target plate is that the acceleration forces are so great upon impact that the entire target, after repeated blows, simply fractures or fails. This failure is not only due to the high G-forces generated internally, but also the fact that uneven stress distribution is created from the center to the periphery of the target plate. With an initial impact force distributed over a small cross-sectional area, a very high bending moment occurs in the center of the plate which seeks to place its bottom surface in tension and the top surface in compression. This bending moment or stress concentration becomes progressively less toward the periphery of the plate.
It is, therefore, another object of this invention to provide a target plate for an acoustic generator wherein the internal stresses are distributed more evenly.
It is a further object of this invention to provide a target for an acoustic signal generator wherein destructive high G-forces are neutralized without interfering with the acoustic efficiency of the target.
Other and further objects of this invention will become apparent from a consideration of the detailed description of the preferred embodiments to follow, taken in conjunction with the drawings and the appended claims.